The present invention relates in general to high frequency mixing circuits, more particularly, to an impedance transforming power combiner operating in a frequency mixing circuit.
Frequency mixing circuits are commonly used in high frequency applications such as hand-held and cellular telephones and portable radios. A frequency mixing circuit downconverts the frequency of a radio frequency (RF) input signal to produce an intermediate frequency (IF) output signal. The RF signal is downconverted by mixing it with a local oscillator (LO) signal to produce the IF signal having a lower frequency equal to the difference between the LO signal frequency and the RF signal frequency.
A conventional mixer may comprise a power combiner for summing the power levels of the RF signal and the LO signal and processing the summation through an impedance matching circuit to the gate of a field effect transistor (FET). The impedance matching circuit provides a matching impedance at the gate of the FET to minimize reflections and associated insertion loss. The LO signal modulates the RF signal at the gate of the FET and produces sum and difference mixing products from the nonlinear transconductance of the FET. The IF signal is obtained by isolating the desired difference frequency (LO.sub.freq -RF.sub.freq) at the drain of the FET.
The power combiner and the impedance matching circuit as practiced in the prior art typically include a network of large inductors and capacitors which consume large physical areas. Inductors require most of the allocated space. The problem is especially acute for integrated circuit applications. It is desirable to simplify the design and minimize the space requirements of the power combiner and impedance matching circuit in the mixer.
Hence, a need exists for an improved frequency mixing circuit having a power combiner and impedance matching circuit requiring less physical area in an integrated circuit.